To increase the storage density of an optical disk, a laser beam for reading and/or writing data from/on it should have a shortened wavelength. Most of DVD players and recorders currently on the market use red semiconductor lasers operating at wavelengths of 660's nm. A red semiconductor laser like this is fabricated by epitaxially growing InGaAlP based compound semiconductors on a GaAs substrate, for example.
Recently, people are spending a lot of time and energy in developing next-generation optical disks that have higher storage densities than DVDs. A light source for each of those next-generation optical disks needs to constantly emit a violet laser beam (falling within the wavelength range of 400's nm), of which the wavelength is even shorter than that of the red ray.
A Group III-V nitride semiconductor, including nitrogen (N) as its Group V element, has a broader bandgap, absorbs or emits light with greater energy, and has a shorter emission wavelength, than a GaAs based semiconductor. In view of this advantage, the nitride semiconductor is expected to be applicable as a material of emitting short-wave light.
Among various nitride semiconductors, a gallium nitride based compound semiconductor (i.e., GaN based semiconductor: AlxGayInzN, where 0≦x, y, z≦1 and x+y+z=1) has been researched so extensively as to put blue and green light-emitting diodes (LEDs) on the market. Also, to further increase the capacities of optical disk drives, semiconductor lasers with oscillation wavelengths of 400's nm are in high demand and semiconductor lasers made of GaN based semiconductors have been researched.
However, unlike the GaAs based semiconductor lasers, those GaN based semiconductor lasers have been fabricated on a sapphire substrate or low-dislocation GaN on sapphire, not on a semiconductor substrate. This is because it has been difficult to make a GaN based semiconductor substrate of quality. But the quality of GaN based semiconductor substrates has been improved just recently to the point that people attempt to fabricate a nitride semiconductor device on a GaN based semiconductor substrate.
FIG. 9 is a perspective view illustrating the structure of a conventional semiconductor laser with a multilayer structure on a GaN substrate. Semiconductor lasers of this type are disclosed in Japanese Patent Application Laid-Open Publications Nos. 11-330622 and 2001-148357, for example. Hereinafter, a method for fabricating the semiconductor laser shown in FIG. 9 will be described with reference to FIGS. 10 through 14.
First, an n-type GaN substrate 301 is prepared as shown in FIG. 10. The n-type GaN substrate 301 is made up of hexagonal single crystals and has a principal surface (i.e., the upper surface) that is a (0001) plane.
Next, as shown in FIG. 11, an n-type nitride semiconductor 303 and a p-type nitride semiconductor 304 are deposited on the n-type GaN substrate 301 by a metalorganic chemical vapor deposition (MOCVD) process. The n-type nitride semiconductor 303 includes an n-AlGaN cladding layer and an n-GaN optical guide layer, which are stacked in this order on the n-type GaN substrate 301. On the other hand, the p-type nitride semiconductor 304 includes a Ga1-xInxN/Ga1-yInyN (where 0<y<x<1) multi-quantum well (MQW) active layer, a p-GaN optical guide layer, a p-AlGaN cladding layer and a p-GaN contact layer, which are stacked in this order over the n-type GaN substrate 301.
Next, as shown in FIG. 12, the upper surface of the p-type nitride semiconductor 304 is patterned into a plurality of ridge stripes, each having a width of about 2 μm. FIG. 12 illustrates a cross section as viewed on a plane that is perpendicular to the resonant cavity direction.
Thereafter, as shown in FIG. 13, each of the ridge stripes made of the p-type nitride semiconductor 304 is covered with an insulating film 305 on right- and left-hand sides. In this case, the upper surface of each ridge stripe is exposed through a striped opening of the insulating film 305. Subsequently, p-electrodes 306 of Ni/Au are formed so as to make contact with the p-type nitride semiconductor 304 at the top of the ridge stripes. On the back surface of the substrate 301, n-electrodes 307 of Ti/Au are formed after the back surface has been polished if necessary.
Next, the substrate 301 is subjected to a first cleavage process in the <11-20> direction of the substrate 301, thereby defining resonant cavity facets as (1-100) planes. More particularly, a lot of bars are made out of a single wafer by the first cleavage process. Each bar has resonant cavity facets that are parallel to the paper of FIG. 13. Thereafter, each bar is split into a number of chips by a second cleavage process. More specifically, each bar is subjected to the second cleavage process in the <1-100> direction, thereby splitting the bar into multiple chips as shown in FIG. 14. The <1-100> direction is perpendicular to the (1-100) planes as the resonant cavity facets. The surfaces that are exposed as a result of the second cleavage process are (11-20) planes that cross both the resonant cavity facets and the principal surface of the substrate at right angles.
In operating the device shown in FIG. 9, the n-electrode 307 is grounded and a voltage is applied from a driver (not shown) to the p-electrode 306. Then, holes are injected from the p-electrode 306 toward the MQW active layer, while electrons are injected from the n-electrode 307 toward the MQW active layer. As a result, a population inversion is produced in the MQW active layer to cause an optical gain. Consequently, a laser beam is produced at oscillation wavelengths of 400s nm.
In making semiconductor laser chips by the above method, however, the second cleavage process for splitting each bar into a number of chips often results in a poor yield. This is a problem caused by the crystal structure of a nitride semiconductor substrate of GaN, for example.
Generally speaking, a (1-100) plane of a hexagonal nitride semiconductor is a crystallographic plane that is easy to cleave. However, a (11-20) plane thereof, which crosses the (1-100) plane at right angles, cleaves less easily than the (1-100) plane. For that reason, in order to define resonant cavity facets of quality for a semiconductor laser that uses a GaN substrate, the first cleavage process is usually carried out in the <11-20> direction to define resonant cavity facets as (1-100) planes and then the second cleavage process is carried out in the <1-100> direction for the purpose of chip splitting. In the cleavage process done in the <1-100> direction, however, the substrate often cleaves in a direction that has deviated from the <1-100> direction by 30 degrees, e.g., in a <2-1-10> direction, thus decreasing the yield.
In order to overcome the problems described above, an object of the present invention is to provide a nitride semiconductor device at a high production yield and a method for fabricating such a device.